The present invention relates to methods and apparatus for driving a power semiconductor device, and in particular, to a method and apparatus for driving a power MOS device as a synchronous rectifier.
Driven by the increasing need for improved efficiency and made practical by the availability of ultra low Rdson power MOSFETS, the replacement of PN or Schottky rectifying diodes by MOSFETs is becoming popular in low voltage applications. A typical example is the automobile alternator. By replacing the diode rectifying bridge (which drops more than two volts) by suitably driven MOSFETs, one can gain 10 to 15% on the overall alternator efficiency. FIG. 1 shows such a prior art system employing MOSFETs in place of diodes.
One of the problems for the designer of such a system is to find a way to drive the FETs in a way that mimics the behavior of diodes, but without the limitation of diodes.
An object of the present invention is to provide a way to drive the rectifying MOSFETs which does not suffer from the limitations of the prior art.
FIG. 2, comprising FIGS. 2A to 2C, shows a known way of implementing the synchronous rectifier MOSFET shown in FIG. 1.
FIG. 2A shows the principle of the circuit, showing one MOSFET; FIG. 2B shows the static operation graphically displaying Id against Vds and FIG. 2C shows waveforms of the circuit of FIG. 2A.
When Vds is positive the body diode of the FET 20 is reverse biased and the MOSFET is off. The operating point is on segment 3 of FIG. 2B. If an AC waveform is applied to the device, the operating point will eventually reach point 1 of FIG. 2B, where the condition Vds=−Von is satisfied. As a result, the output of the Schmidt trigger 10 will go high and the power MOSFET 20 will be turned on. The operating point will move to the segment 4 of FIG. 2B. Eventually the AC waveform will become positive and the operating point will reach point 2. The condition Vds>Voff is met and the Schmidt trigger will turn off the power MOSFET.
A practical application of such a circuit is made difficult because the threshold has to be very tightly controlled, requiring very low offset comparators, in a usually noisy environment. In a typical application, one would use MOSFETs of 1 milliohm for currents around 100 A. It follows that a 1 millivolt offset will create 1 A of undesirable negative current at point 2. A root of the problem is that the designer is trying to reproduce a zero current crossing detection by sensing the voltage across a device with practically zero parasitic resistance.
Accordingly, it is an object of the present invention to provide an improved circuit and method for operating a power MOS device as a synchronous rectifier.